A fire that caused extensive damage to a Labour Party office is being treated as deliberate by police.

The building was empty at the time and no-one was hurt but it could cost thousands of pounds to repair the damage, officers said.

The blaze broke out at the office in Broadmeadow Industrial Estate, Dumbarton, at around 10pm on Sunday. The fire service and police have been investigating the fire and are treating it as wilful.

Officers are particularly keen to trace two men who were seen riding bikes in the area shortly before the fire broke out.

Detective Inspector David Moran said: "Setting this fire was utterly irresponsible, reckless and very dangerous. Thankfully there was no-one in the office at the time of the fire, however, the office was very badly damaged with a bill that will no doubt run into thousands of pounds to repair.

"There are also a number of other small businesses in the area which could have suffered if the fire had taken hold.

"I am particularly keen to speak to or identify two men seen riding mountain bikes in the area about 30 minutes prior to the fire being discovered and who may have important information.

"Additionally I would ask that anyone who has any information about the persons responsible for setting this fire to make contact with us."